The testing of motor vehicles on a stationary testing stand involves the problem of how to exactly simulate the road resistance that occurs when in its actual operation a vehicle travels on a road. Conventionally, so-called roller testing stands are used for the required simulation. The vehicle or vehicle components, such as an axle and wheel set, are arranged on these roller testing stands in a quasi-stationary condition but in a force transmitting manner. Since the vehicle or a vehicle component on the testing stand does not actually move, and since the mass of the testing stand normally does not correspond to the mass of the vehicle or the test sample, it is necessary to simulate at least a difference mass corresponding to the difference between the mass of the vehicle (M.sub.F) and the mass of the testing stand (M.sub.P).
It is further desirable that such stationary testing stands are capable of simulating any other road resistances by simulation of an additional moment or torque moment. Thus, it is desirable to simulate in the testing stand forces which normally are effective on the vehicle if the vehicle is travelling on a road. For this purpose all translatory, physical magnitudes or values such as the vehicle mass, the force applied by the vehicle etc., are transformed by calculation into rotatory values or magnitudes with the aid of the radius of the testing rollers forming part of the testing stand. The resulting difference mass between the actually available testing stand mass and the vehicle mass to be realized is then simulated electrically with the aid of a load generator coupled to the testing rollers. The difference mass to be realized or simulated is controlled in a dynamic closed loop control circuit causing the load generator to apply a respective simulated load or torque moment to the testing rollers. Conventional circuits of this type are subject to different problems depending on the type of mass simulation method employed. Depending on the small or large positive difference masses, stability problems and closed loop control accuracy problems do occur in conventional testing stands.
German Patent Publication (DE-PS) 1,105,637 (Waller) published Apr. 27, 1961 discloses a so-called differentiating mass simulating method for vehicle testing stands. According to the Waller method the difference mass is simulated by an electric load generator coupled to the running rollers of the testing stand which includes a Ward-Leonard set as a load generator. More specifically, the difference mass to be simulated is electrically generated by the application of an additional torque moment to the axis of the testing stand rollers with the aid of a d.c. motor-generator. For this purpose the load or torque moment to be applied is ascertained indirectly through the acceleration effect of the testing stand mass. The acceleration effect is in turn ascertained by an r.p.m. pick-up such as a tachometer or generator, the output voltage of which is proportional to the r.p.m. of the load application output shaft of the load simulating generator. The output voltage of the r.p.m. pick-up is differentiated to thereby produce a signal proportional to the acceleration. A calculating circuit ascertains from the acceleration proportional signal a load or torque moment which corresponds to the rated or reference torque value of the mass component to be simulated, whereby the calculating circuit also takes into account the known vehicle inertia moment and the known testing stand inertia moment. The rated or reference value is then used to produce a closed loop control signal for controlling the load generator for adjusting the load or torque moment applied to the testing stand rollers by the load generator. This type of closed loop differentiating feed-back control results in a non-stable closed loop control characteristic, especially when there are large mass differences between the mass of the testing stand and the mass of the sample such as a vehicle on the testing stand.
German Patent Publication (DE-PS) 2,738,325 C2 (Fegraus et al.) published on Mar. 2, 1978 describes a so-called integrating mass simulation method, wherein the mass to be simulated is derived from the integration of the measured torque moment and from the known inertia masses of the vehicle and of the testing stand. The known system includes a calculating circuit which calculates from the measured torque moment signal by integration a rated or reference r.p.m. which then serves for controlling the load application generator in closed loop fashion. Thus, the mass simulation is generated by the closed loop r.p.m. control of a d.c. motor generator functioning as a load generator. The system of Fegraus et al. avoids the drawbacks of the differentiating mass simulating method according to Waller. However, this integrating feed-back control method also results in an unstable closed loop control characteristic, especially for small mass differences between the testing stand mass and the vehicle mass.
A combination of the differentiating simulating method with the integrating simulating method would require two separate closed loop control systems. Further, practical experience has shown that especially in the area of the control characteristic where unfavorably large or small mass differences occur, it is not possible to achieve a satisfactory closed loop control accuracy or quality. This is particularly true in areas of large mass differences between the testing stand mass and the vehicle mass.